1. Field of the Invention
This invention relates to apparatus which provides a direct load path for seismic forces between the shell and internals of a pressure vessel despite variations in the spacing between the shell and the internals due to differences in thermal expansion of the components and to expansion of the shell under pressure.
2. Prior Art
Nuclear steam generators and other pressure vessels must be able to withstand seismic events without damage. The accelerations in these events can create loads on the internals, in the case of steam generators, the tube bundle and supports, which are transmitted to the shell. Any gaps which occur between the shell and the internals lead to higher loads being imposed on the internals than if there were no gaps due to dynamic impacting. For current model steam generators, radial gaps of only 0.10 inches result in a 100% increase in load to the tube supports over the no-gap condition. Reducing the load requires a device which can move, since the gap size varies during heatup and cooldown of the vessel due to differential thermal expansion of the shell and the internals and due to pressure expansion of the shell. Under normal conditions in the steam generator of a pressurized water reactor (PWR) power plant, the device is submerged in slightly subcooled water at temperatures near 520.degree. F., and must reliably survive the 40-year design life of the steam generator.
Under current practice, tube support plates which support the tube bundle in a PWR steam generator are braced in the radial direction by a series of angularly spaced radially-directed elements. These elements include a support block welded into the wrapper. Jacking screws are inserted through threaded holes in the jacking block to align and support the wrapper and are then welded in place. The tube support plates are then inserted and wedges are driven in the unheated shop condition between the tube supports and the jacking block and the wedges are welded to the wrapper. When heating and pressurizing of the steam generator occur, gaps due to thermal mismatches may occur between the tube support plates and the wedges, and gaps due to pressure and thermal expansion differentials may occur between the jacking screws and the shell which combine to lead to higher loading on the internals from seismic events.